Planar flexure members and actuators using them

ABSTRACT

A planar flexure member for resisting rotation about a central axis thereof includes, in various embodiments, a central portion comprising a plurality of attachment points; and at least one serpentine flexure arm extending from the central portion in a plane. The arm(s) terminate in an arcuate mounting rail that includes a series of attachment points. The rails are positioned in opposition to to each other to partially define and occupy a planar circular envelope radially displaced from but surrounding the central portion of the flexure member. A portion of the serpentine arms may extend to (or substantially to) the envelope between the mounting rails.

TECHNICAL FIELD

The present invention relates to elastic flexure elements and actuatorsemploying these elements for use, for example, in robotic applications.

BACKGROUND

Industrial robots perform a variety of tasks involving the movement andmanipulation of various objects. A typical industrial robot as used,e.g., in a manufacturing environment, may have one or more arms equippedwith grippers that allow the robot to pick up, transport, and manipulateobjects. A key mechanical requirement for industrial is the ability togenerate large but precise forces and torques while maintaining overallcontrol stability. These torques and forces are generated by actuators,i.e., motors responsive to control signals to apply a commanded torque,which is transmitted mechanically to a load either directly (whererotational actuation is required) or via a linear conversion element,such as a lead screw (when linear force is required).

Stiff actuators can exert large forces from small joint displacements,and permit high-bandwidth force control and precise position control.But stiffness makes force control difficult. Because of the importanceof force control in robotic applications, stiffness and the attendantbandwidth is typically sacrificed to achieve better force control. Oneapproach is to utilize an elastic element in series with the actuator.Elasticity has the effect of making the force control easier, as largerdeformations are needed to exert a given force relative to a stiffactuator. robot. In effect, the elasticity allows force to be controlledvia position rather than directly, which improves accuracy andstability, and reduces noise.

Designing series elastic elements for robotic applications can bechallenging due to space constraints, the need to withstand large andrepeated applied torques without slippage or wander, and the need forrepeatable but economical manufacture. In a rotational elastic element,for example, the design must incorporate components with sufficientlength to provide the desired elasticity (since stiffness variesinversely with the cube of a component's length), but must also providea secure mounting frame to avoid slippage. Because the frame typicallydefines the outer envelope of the elastic element, it imposes a limit onthe amount of internal length that may be employed.

SUMMARY

The present invention provides, in various embodiments, a planar flexuremember for resisting rotation about a central axis thereof that affordsgreater compliance than conventional designs. In various embodiments,the flexure member comprises a central portion comprising a plurality ofattachment points; and at least two serpentine flexure arms extendingoppositely and symmetrically from the central portion in a plane, eachof the arms terminating in an arcuate mounting rail, the mounting railseach comprising a plurality of attachment points and being positioned inopposition to to each other to partially define and occupy a planarcircular envelope radially displaced from but surrounding the centralportion, a portion of the serpentine arms extending substantially to theenvelope between the mounting rails.

In some embodiments, the serpentine arms have a varying thickness with athinnest portion thereof at the envelope. The arms and the centralportion may have a unitary height at least equal to the width of thearms at a narrowest portion thereof. For example, the ratio of height towidth may be at least 2. In other embodiments, the arms and the centralportion have a non-unitary height.

The flexure member may be made of titanium or other suitable metal (orother material). In some implementations, the arms (or portion thereof)have an I-beam cross-section. The arms may alternatively or in additioninclude voids along a neutral bending axis thereof.

In another aspect, the invention pertains to a planar flexure member forresisting rotation about a central axis thereof. In various embodiments,the flexure member includes a central portion comprising a plurality ofattachment points; and at least one serpentine flexure arm extendingfrom the central portion in a plane and terminating in an arcuatemounting rail having a plurality of attachment points.

In still another aspect, the invention relates to a rotary actuator. Invarious embodiments, the actuator comprises a motor configured forrotation about an actuation axis; and a planar flexure member having acentral output portion mechanically coupled to a load and at least twoserpentine flexure arms extending oppositely and symmetrically from thecentral portion in a plane, each of the arms terminating in an arcuatemounting rail having a plurality of attachment points for mounting tothe motor, the mounting rails being positioned in opposition to eachother to partially define and occupy a planar circular envelope radiallydisplaced from but surrounding the central portion, a portion of theserpentine arms extending substantially to the envelope between themounting rails.

In some embodiments, the serpentine arms have a varying thickness with athinnest portion thereof at the envelope. The arms and the centralportion may have a unitary height at least equal to the width of thearms at a narrowest portion thereof. For example, the ratio of height towidth may be at least 2. In other embodiments, the arms and the centralportion have a non-unitary height. The flexure member may be made oftitanium or other suitable metal (or other material). In someimplementations, the arms (or portion thereof) have an I-beamcross-section. The arms may alternatively or in addition include voidsalong a neutral bending axis thereof.

In some embodiments, the actuator has an actuation axis coaxial with anoutput axis. In other embodiments, the actuator has an actuation axisparallel to and offset with respect to an output axis, or oblique withrespect to an output axis.

The term “substantially” or “approximately” means±10% (e.g., by weightor by volume), and in some embodiments, ±5%. The term “consistsessentially of” means excluding other materials that contribute tofunction, unless otherwise defined herein. Nonetheless, such othermaterials may be present, collectively or individually, in traceamounts. Reference throughout this specification to “one example,” “anexample,” “one embodiment,” or “an embodiment” means that a particularfeature, structure, or characteristic described in connection with theexample is included in at least one example of the present technology.Thus, the occurrences of the phrases “in one example,” “in an example,”“one embodiment,” or “an embodiment” in various places throughout thisspecification are not necessarily all referring to the same example.Furthermore, the particular features, structures, routines, steps, orcharacteristics may be combined in any suitable manner in one or moreexamples of the technology. The headings provided herein are forconvenience only and are not intended to limit or interpret the scope ormeaning of the claimed technology.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be more readily understood from the followingdetailed description of the invention, in particular, when taken inconjunction with the drawings, in which:

FIG. 1 schematically illustrates a representative actuator systememploying an embodiment of the invention.

FIGS. 2A and 2B illustrate, respectively, perspective and sectionalviews of a planar flexure member in accordance with embodiments of thepresent invention. The width w and length L of the flexure arms of themember are indicated on FIG. 2A, as is the neutral bending axis of theflexure arm (dashed line). The thickness h of the central portion isindicated on FIG. 2B, as is the thickness h_(a) of the flexure arms.

FIG. 2C is a sectional view of a planar flexure member in accordancewith embodiments of the present invention in which the thickness h ofthe central portion is the same as the thickness h_(a) of the flexurearms.

FIG. 2D is a sectional view, taken along line 2D-2D in FIG. 2A, of aflexure arm having an I-beam cross section in accordance withembodiments of the present invention.

FIGS. 3A and 3B are plan and sectional views, respectively, of arepresentative deployment of the flexure member shown in FIGS. 2A and 2Bin which the actuation axis of the actuator in which the flexure memberis deployed is coaxial with the output axis. Some components are omittedfor clarity in FIG. 3A.

DETAILED DESCRIPTION

FIG. 1 illustrates the basic components of an actuator system 100 thatincorporates a flexure element in accordance herewith. The system 100includes a torque-generating device, i.e., a motor 110, which mayoptionally be geared via a gearbox 112 (which lightens the system byfacilitating use of a smaller motor 110 operating at higher speeds). Thegearbox 112 may be integral to or separate from the motor 110. Atorsional spring element 114 is linked in series with the output of thegearbox 112, or if no gearbox is employed, directly with the motor 110.The load 116 to be acted upon by the actuator system 100 is linked inseries to the other end of spring element 114. The spring element 114thereby introduces at the interface between the actuator 100 and theload 116 a series elasticity that affords precise control of the forceapplied to the load. The spring element 114 may be linked to the load116 through a low-backlash transmission element (not shown) if desired.

In a robot environment, the axial distance between the actuator system100 and the load 116 may be tightly constrained, limiting the thicknessof the spring element 114. The radial extent of the actuator system 100may also be highly constrained, limiting the envelope diameter of thespring element. Hence, it is essential to pack the desired degree ofstiffness into a small spatial region, while at the same time providingfor sufficiently secure mounting of the spring element 114 to thegearbox 112 and the load 116 (or other mechanical output) to avoidslippage and wander.

A representative elastic element fulfilling these contradictoryconstraints is shown in FIGS. 2A and 2B. The flexure member 200 is aplanar structure having generally flat opposed surfaces, the visible oneof which is indicated at 205. A central portion 215 includes a pluralityof attachment points 217—i.e., mounting holes arranged in a generallycircular configuration and typically spaced equidistantly apart. Theattachment points 217 accommodate screws or other fasteners that securethe flexure member 200 to the actuator motor or gearbox as describedearlier.

Emanating from the central portion 215 are a pair of serpentine flexurearms 220 a, 220 b, which extend oppositely and symmetrically from thecentral portion 215 in a plane. Although two arms 220 are shown, itshould be understood that configurations utilizing a single arm 220, aswell as more than two arms 220, are within the scope of the invention.The width w of the arms 220 (which may change along the length of thearms), as well as the length L of the arms 220 are indicated in FIG. 2A.In the illustrated embodiment, each of the arms 220 a, 220 b terminatesin an arcuate mounting rail 225 a, 225 b. Each of the mounting rails 225includes a plurality of attachment points 217 (mounting holes, onceagain, in the illustrated embodiment) that facilitate attachment of theflexure member 200 to the load or the drive. As best seen in FIG. 3A,described in greater detail below, the mounting rails 225 are positionedin opposition to define, along with the outer curved segments of theflexure arms 220, a substantially circular outer envelope for stabilityand symmetry of rotative force transmission. Because the mounting rails225 occupy only a portion of the circular envelope, the flexure arms mayextend outwardly so that the outer curved edges meet or approach theenvelope. In this way, the lengths of the flexure arms 220 may bemaximized within a limited circular area—i.e., their lengths are notconstrained to fit inside a fully circular mounting collar.

With reference to FIG. 2B, the flexure member 200 has a height h, whichdepends, in various embodiments, on the size of the actuator.Furthermore, although the flexure member 200 is planar, the height h mayvary—that is, different regions of the flexure member 200 may havedifferent thicknesses. For example, FIG. 2B depicts an embodiment inwhich the height h of the central portion 215 of flexure member 200 isgreater than the height h_(a) of the arms 200. A representative range ofheights is 2.5 to 9 mm. Where the height varies, a typical configurationhas the thinnest (lowest h_(a)) portion of the arms 220 at the outeredges thereof. Where the arms 220, the central portion 215 and the rails225 have a unitary height, that height may be at least equal to thewidth of the arms at a narrowest portion thereof, representativelyindicated at 230 in FIG. 2A; in one illustrative implementation, theratio of height to width is 2:1. FIG. 2C depicts an embodiment in whichthe height h of the central portion 215 of flexure member 200 is equalto the height h_(a) of the arms 200, i.e., the central portion 215 andthe arms 200 have a unitary height.

The arms 220 provide the elasticity of the flexure member 200. That is,as the central portion 215 is rotated, rotary force is transmitted tothe arms 220. The outside of the flexure member 200 is attached to thegearbox 112 (see FIG. 1). The arms 220 elastically deform to a degreedependent on the torque applied to the central portion 215 and thereaction force of the load. The elasticity of the flexure member 200depends on the modulus of the material from which the flexure member isfabricated as well as the lengths and thicknesses of the arms 220. Inparticular, each of the arms 220 may be approximately modeled as acantilever beam with a stiffness k given by

$k = \frac{E\; h_{a}w^{3}}{4\; L^{3}}$where E is the Young's modulus of the flexure member 200, w is thecross-sectional width (radial dimension) of the arm shown in FIG. 2A,h_(a) is the height (z-axis) of the arm shown in FIGS. 2B and 2C, and Lis the length of the arm (from the central portion 215 to the mountingrail 225) shown in FIG. 2A.

Because of this relationship, z-axis arm thickness h_(a) can be tradedoff against arm width w in the xy plane of the flexure member 200. Ifthickness is constrained by space limitations or machinability, in otherwords, a given reduction in thickness can be compensated for by a cubicincrease in arm width in order to maintain the same stiffness. Althoughthe cubic relationship implies a large areawise increase in the armfootprint to achieve a thickness reduction, in fact this increase isreadily accommodated by the serpentine configuration, which leavessubstantial open space within the envelope of the flexure member200—space that is further increased by the limited-circumferencemounting rails 225, which allow the outer edges of the arms 220 to bemaximally spaced from the central portion 215. Other weight-reductionstrategies may also be employed. For example, the arms may be shapedwith an I-beam cross-section, as shown in FIG. 2D, to reduce the amountof material needed to achieve a given stiffness, or material may beremoved along the neutral axis of bending 250 (e.g., voids or holes 260may be formed along the neutral axis 250, as shown in FIG. 2A).

Indeed, wider arms can aid manufacturability, since narrow features canbe difficult to fabricate. Typical approaches used in the manufacture ofplanar flexures include stamping, water-jet cutting, laser cutting, andmachining Stamped parts can exhibit inferior edge quality and thereforedurability limitations, and it can be difficult to retain complexfeature shapes following heat treatment; hence slender, curved armsegments may be incompatible with stamping as a fabrication option.Water jet/laser cutting cutting generally has a low-end dimensionalcontrol of about 0.005″ for materials suitable for flexure members ascontemplated herein, and for flexures designed for small operatingtorques, this variation translates into very large stiffness variations,since stiffness varies with the cube of the dimensional error.Additionally, the cost of water jet/laser cutting is faily high comparedwith processes like extruding and slicing, and does not ramp to volumeproduction easily. If desired, a finishing technique maybe employed toadjust the final mechanical properties of the flexure member 200. Forexample, peening (e.g., shot peening) is frequently used to introducesurface residual compressive stresses and thereby increase thedurability of metal parts.

In general, an extrusion process followed by slicing into planar flexureelements is cost-effective and well-suited to embodiments of the presentinvention. A preferred material for the flexure element 200 is titanium,particularly when the flexure element is affixed to an aluminum loadand/or rotor. The coeffiecient of friction between aluminum and titaniumis higher than between steel and aluminum, reducing the possibility thatthe bolted joint will slip. Although a titanimum flexure requires morematerial, the volume offset does not outweigh the density reductiontitanium offers, and the net result is a lighter flexure. Titanium has anatural endurance limit in the same way steel does (though unlike manyother materials) and therefore is well suited to elastic applications.Titanium has 60% of the stiffness of steel, which means that the flexurearms need to be a bit thicker relative to steel, reducing theirsensitivity to tolerance variation. It should be noted that more thanone flexure in accordance herewith may be stacked in variousconfigurations to achieve balanced loading and the required torquedeflection.

FIGS. 3A and 3B show the flexure element 200 coupled to a load in arepresentative mechanical environment. The load itself (not shown, butwhich may be, for example a robot arm) is coupled via bolts 315 passingthrough the central mounting holes of the flexure element 200. Acircular frame 320 is mechanically coupled to the central portion of theflexure element 200 via cross roller bearings or a similar system. As iswell understood in the art, crossed roller bearings comprise outerrings, inner rings, and rolling elements; they can also be metalspacers. Due to the crossed arrangement of the rolling elements, suchbearings can support axial forces from both directions as well as radialforces, tilting moment loads and combinations of loads with a singlebearing position. The outer rails 325 of the flexure element 200 aresecured to a source of rotary power, such as a harmonic drive 330. Thus,the flexure element 200 transmits torque from the drive 330 to thesystem output, acting as a spring therebetween, and the actuation axisof the flexure element 200 and the output axis of the system are coaxial(indicated as axis 340 on FIGS. 3A and 3B).

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. In particular,embodiments of the invention need not include all of the features orhave all of the advantages described herein. Rather, they may possessany subset or combination of features and advantages. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. A planar flexure member for resisting rotationabout a central axis thereof, the flexure member comprising: a centralportion comprising a plurality of attachment points; and at least twoserpentine flexure arms extending oppositely and symmetrically from thecentral portion in a plane, each of the arms terminating in an arcuatemounting rail, the mounting rails each comprising a plurality ofattachment points and being positioned in opposition to to each other topartially define and occupy a planar circular envelope radiallydisplaced from but surrounding the central portion, a portion of theserpentine arms extending substantially to the envelope between themounting rails.
 2. The flexure member of claim 1, wherein the serpentinearms have a varying thickness with a thinnest portion thereof at theenvelope.
 3. The flexure member of claim 1, wherein the arms and thecentral portion have a unitary height, the height being at least equalto a width of the arms at a narrowest portion thereof.
 4. The flexuremember of claim 3, wherein a ratio of the height to the width is atleast
 2. 5. The flexure member of claim 1, wherein the arms and thecentral portion have a non-unitary height.
 6. The flexure member ofclaim 1, wherein the flexure member is made of titanium.
 7. The flexuremember of claim 1, wherein at least a portion of the arms has an I-beamcross-section.
 8. The flexure member of claim 1, wherein at least aportion of the arms has voids along a neutral bending axis thereof.
 9. Arotary actuator comprising: a motor configured for rotation about anactuation axis; and a planar flexure member having a central outputportion mechanically coupled to a load and at least two serpentineflexure arms extending oppositely and symmetrically from the centralportion in a plane, each of the arms terminating in an arcuate mountingrail having a plurality of attachment points for mounting to the motor,the mounting rails being positioned in opposition to each other topartially define and occupy a planar circular envelope radiallydisplaced from but surrounding the central portion, a portion of theserpentine arms extending substantially to the envelope between themounting rails.
 10. The actuator of claim 9, wherein the serpentine armshave a varying thickness with a thinnest portion thereof at theenvelope.
 11. The actuator of claim 9, wherein the arms and the centralportion have a unitary height, the height being at least equal to awidth of the arms at a narrowest portion thereof.
 12. The actuator ofclaim 9, wherein a ratio of the height to the width is at least
 2. 13.The actuator of claim 9, wherein the arms and the central portion have anon-unitary height.
 14. The actuator of claim 9, wherein the flexuremember is made of titanium.
 15. The actuator of claim 9, wherein atleast a portion of the arms has an I-beam cross-section.
 16. Theactuator of claim 9, wherein at least a portion of the arms has voidsalong a neutral bending axis thereof.
 17. The actuator of claim 9,wherein the actuator has an actuation axis coaxial with an output axis.